1. The Field of the Invention
The present invention relates to detecting targets in roadway intersections.
2. The Relevant Technology
The use of traffic sensors for the actuation of traffic signal lights located at the intersection of roadways is quite common. Generally, such traffic sensors can provide input used to properly actuate traffic control devices in response to the detection or lack of detection of vehicles. For example, traffic sensors can enable a traffic control device to skip unnecessary signal phases, such as, for example, skipping a left hand turn phase when no vehicles are detected in a corresponding left hand turn lane. Traffic sensors can also enable a traffic signal to increase green light duration for major arterials by only signaling the green light in the minor cross streets when vehicles are detected on the minor cross streets and thus minimizing the red light for a major arterial. Thus, traffic sensors assist in properly actuating a signalized intersection to improve traffic flow. In addition to the actuation of signalized intersections of roadways for automobile traffic, traffic sensors are also used for the actuation of intersections of a roadway for automobile traffic with a railway.
Unfortunately, the cost of traffic sensors and cost of corresponding installation can be relatively high. Thus, traffic sensors and related costs can become a significant expenditure for municipalities. The high installation costs arise at least in part from the need to install sensors for every approach to an intersection.
Typically, traffic signal lights have been actuated using inductive loop detectors embedded in the roadway. Inductive loop detectors are very expensive to install since lane closures are necessary. The high cost is compounded, especially for multi-lane roadways, since at least one inductive loop detector is required for each detection zone (e.g., left hand turn lane detection zones, through lane detection zones, and right hand turn lane detection zones). Furthermore, inductive loop detector technology is often unreliable and inductive loop detectors require a great deal of calibration.
Video detectors are also used in some traffic signal actuation systems. To facilitate traffic signal light actuation, a video camera is placed high above a signal arm such that the video camera's view covers one approach to the intersection. The video signal from the camera is digitally processed to create detections in the defined zones. Using video detectors an intersection can be monitored on a per approach basis (that is all the lanes of an approach), as opposed to the per detection zone basis used with inductive loops. However, at least one camera per approach is required. Since a dedicated mounting arm is often necessary and at least one camera per approach is required, the installation of a video detector system can also be expensive and time consuming.
Microwave detectors have also been used in intersections to provide detection coverage over limited areas. At least one microwave detector has a limited degree of mechanical and electrical steering. However, similar to video detectors, one microwave detector per approach is required and the coverage is typically over a small portion of the intersection. Further, manual configuration is needed to ensure that the proper detection zones from each sensor are wired to the proper input in the traffic controller.
Other microwave sensors have included multiple receive antennas but have included only a single transmit antenna that has a very broad main beam or even may be an omni-directional antenna. Systems that employ only one broad beam or omni-directional transmit antenna typically cannot achieve an appropriately reduced side lobe power level. Furthermore, these single transmit antenna systems typically suffer from widening of the mainlobe.
Acoustic sensors have also been used in intersections to cover limited detection zones. However, these sensors also require one unit per approach. Therefore intersection traffic detection products that reduce the number of sensors and sensor installations required would be advantageous.